Lead-acid storage batteries comprise several galvanic cell elements each encased in separate compartments of a substantially leak-proof, thermoplastic container containing sulfuric acid electrolyte. Each cell element typically comprises at least one plate-like, positive electrode (i.e., positive plate), one plate-like negative electrode (i.e., negative plate) and a porous separator (e.g., a thin microporous sheet and/or absorbent glass mat) therebetween. Multi-plate cell elements are commonplace and comprise a stack of alternating positive and negative polarity plates interleaved one with the other and the separators. The plates themselves each comprise a conductive substrate which supports an electrochemically active material thereon and conducts electrical current substantially uniformly therethrough. In Pb-acid batteries, the plates comprise a leady active material (i.e., PbO.sub.2 for the positive plates and Pb for the negative plates) pasted onto a reticulated Pb-alloy (e.g., Pb-Ca-Sn or Pb-Sb) grid substrate. A lug projects from each grid and serves to electrically couple its associated plate to other electrical components of the battery. For example, aligned lugs of like polarity plates of a multi-plate cell element are commonly electrically coupled one to the other and to intercell connectors or terminals by a so-called plate strap which is typically burned to, or cast about, the plates' lugs. Heretofore, essentially four techniques have been proposed to make such plate straps.
In the first technique, comb-like iron tooling is interdigitated with the lugs to form a mold around and between the lugs, a pre-cast plate strap having a plurality of toes is interdigitated with the plate lugs in the mold and burned to the plate lugs by means of a gas torch applied directly thereto. Thereafter, the tooling is withdrawn. This technique has been used commercially for many years.
In the second technique, upstanding plate lugs are enclosed in a mold similar to that described above and molten lead poured into the mold to form the plate strap. This technique never achieved widespread, if any, commercial use presumably owing to the inability to reliably implement such a process on a production scale. In this regard, metal which was poured into the mold at a single site was expected to spread uniformly throughout the mold (i.e., in and around the upstanding plate lugs) and still bond well to the lugs. However, non-uniform distribution of the lead in the mold as well as non-uniform cooling of the melt usually occurred and resulted in poor quality, high resistance connections between the plate lugs and plate strap.
A third technique is a variation of the second wherein an open-topped mold is first filled with a predetermined amount of molten lead and thereafter the plate lugs of an inverted cell element are immersed therein. The solidified strap is subsequently removed from the mold, the cell element returned to its upright position and finally inserted into a battery container. The aforesaid third technique substantially eliminated the non-uniform flow, cooling and heat distribution problems of the second technique and has been used commercially for many years. However, it too has disadvantages. In this regard, commercial practice of this technique requires the use of a melting/holding furnace containing a large supply of molten lead ready for pouring, as well as an associated plumbing network of melt delivery pipes, valves, nozzles, etc. This complicated assemblage of melt handling equipment is cumbersome, requires considerable maintenance and has to be kept hot at all times even when plate straps are not being cast. The heat required to melt and keep large quantities of lead molten, as well as keep the melts' delivery plumbing hot, not only results in a costly consumption of energy but radiates into the work area making it a less desirable operator working environment. Moreover, the maximum practical temperature useful with such prior "cast-on-strap" techniques was effectively limited to about 850.degree. F. above which untoward oxidation of the molten lead occurs which tends to cause equipment fouling and result in oxide inclusions in the casting. Moreover, experience has shown that the equipment commercially available for metering and delivering the lead into the molds by those techniques is not capable of consistently casting plate straps of the exact same size. Rather, the mass of the plate straps varies significantly one from another over the course of a production run. As a result, in order to insure that each and every plate strap has at least the minimum amount of lead necessary for current conduction and strength it is common practice to purposely set the delivery equipment to dispense more lead (often as much as 30% more) into the molds than is theoretically necessary. Hence, many straps are cast with more lead than needed which results not only in heavier batteries but considerable excess cost.
The fourth technique is described in U.S. Pat. Meadows et al No. 4,742,611, which is incorporated herein by reference, and wherein plate lugs in one cell element are joined to each other and to plate lugs in the next adjacent cell element by substantially oxide-free, arc-melted, molten lead cast about the lugs and through an open topped slot formed in the upper edge of an intercell partition which separates the two cell elements from each other. The lugs themselves are shielded from the arc while being bathed in a heated non-oxidizing gas emanating from the arc generator which reduces oxide formation and aids in maintaining the temperature of the melt zone. While the technique of Meadows et al No. 4,742,611 makes an excellent low resistance connection between the lugs and the cell elements, the slot/opening above the intercell connecting portion of the straps needs to be sealed closed to prevent electrolyte leakage between adjacent cells. This sealing/closure step adds to the complexity and cost of the process and provide another site where possible leakage can occur.
It is an object of the present invention to eliminate the extra slot-sealing/closure step required by Meadows et al while otherwise providing all of the benefits of such process. This and other objects and advantages of this invention will become more readily apparent from the detailed description thereof which follows.